Conventionally, a spark ignition type internal combustion engine detects an ion current generated in a combustion chamber after ignition by using a spark plug and detects an operation state of the internal combustion engine such as knocking and combustion limit from magnitude and generation time of the detected ion current to adjust ignition timing or correct a fuel injection amount on the basis of the detection result. In detecting the ion current by using the spark plug, if the spark plug has no defect, the ion current can be detected at each ignition.
Generally, in the spark plug, there may occur the state called as smoldering in which carbon contained in soot generated by burning an air-fuel mixture is adhered to an electrode or an insulator in the vicinity of the electrode. When smoldering is generated in this manner, in detecting the ion current, a leak current is overlapped on the ion current. In a high rotational operation state with short ignition interval, when the ion current is detected before the leak current overlapped on the ion current disappears, it is determined as if the leak current continues. Thus, it may be wrongly determined that the sparkplug is short-circuited. For this reason, it is necessary to detect smoldering and discriminate smoldering and short-circuit of the spark plug.
In consideration with such circumstances, for example, as described in Patent document 1, by forbidding determination of smoldering of the spark plug when the engine rotational speed of the internal combustion engine falls within a predetermined speed range, smoldering is not determined in the state where whether smoldering or short-circuit of the spark plug cannot be determined, specifically, where ignition timing of one cylinder overlaps a period during which the ion current is detected in the other cylinder.
Patent document 1: Unexamined Patent Publication No. 2004-108298